Thermoelectric devices for cooling and heating and the generation of electricity have been known for many years; however, their use has not been cost competitive except for limited applications because of the lack of thermoelectric materials having the needed thermoelectric properties.
A good thermoelectric material is measured by its "figure of merit" or Z, defined as EQU Z=S.sup.2 /.rho.K
where S is the Seebeck coefficient, .rho. is the electrical resistivity, and K is the thermal conductivity. The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or EQU S=V/(T.sub.h -T.sub.c).
Therefore, in searching for a good thermoelectric material, we look for materials with large values of S, and low values of .rho. and K.
Thermoelectric materials currently in use today include the materials listed below with their figures of merit shown:
______________________________________ Thermoelectric Material Peak Zeta, Z (at temperature shown) ZT ______________________________________ Lead telluride 0.9 .times. 10.sup.-3 /.degree.K. at 500.degree. K. 0.9 Bismuth telluride 3.2 .times. 10.sup.-3 /.degree.K. at 300.degree. K. 1.0 Silicon germanium 0.8 .times. 10.sup.-3 /.degree.K. at 1100.degree. K. 0.9 ______________________________________
Workers in the thermoelectric field have been attempting to improve the figure of merit for the past 20-30 years with not much success. Most of the effort has been directed to reducing the lattice thermal conductivity (K) without adversely affecting the electrical conductivity.
Experiments with superlattice quantum well materials have been underway for several years. These materials were discussed in an paper by Gottfried H. Dohler which was published in the November 1983 issue of Scientific American. This article presents an excellent discussion of the theory of enhanced electric conduction in superlattices. These superlattices contain alternating conducting and barrier layers. These superlattice quantum well materials are crystals grown by depositing semiconductors in layers whose thicknesses is in the range of a few to up to about 100 angstroms. Thus, the layers can be as thin as only a few atoms thick. There has been speculation that these materials might be useful as thermoelectric materials. (See articles by Hicks, et al and Harman published in the Proceedings of 1992 1st National Thermoelectric Cooler Conference Center for Night Vision & Electro Optics, U.S. Army, Fort Belvoir, Va. FIG. 1 has been extracted from the Hicks paper and is included herein as prior art. It projects theoretically very high ZT values as the layers are made progressively thinner.) The idea being that these materials might provide very great increases in electric conductivity without adversely affecting the Seebeck coefficient or the thermal conductivity. Harmon of Lincoln Labs, operated by MIT has claimed that he is close to producing a superlattice of layers of (Bi,Sb) and Pb(Te,Se), but to the best of Applicants' knowledge, he has not been successful in producing quantum wells. He claims that his preliminary measurements suggests ZTs of 3 to 4 are possibly feasible. Most of the thermoelectric efforts to date with superlattices have involved alloys such as BeTe which are known to be good thermoelectric materials for cooling, many of which are difficult to manufacture as superlattices because the stoichiometry of the alloys have to be very carefully controlled which is very difficult in vapor deposition techniques. Bulk SiGe is not a good thermoelectric material at low temperatures. Superlattices tend to diffuse at high temperatures and lose their superlattice qualities.
Researchers investigating opto-electronic properties of multilayer quantum well structures have considered the effects of strain in the layers. These researchers (e.g., Abstrater, et al., Phy. Lett. S4, 2441 (1985)) report increased electron mobility due to the strain effect.
What is needed are thermoelectric materials with improved ZT values which permit a simplified manufacturing process.